1. Field of the Invention
This invention relates to the use of supersensitizers in photographic emulsions.
2. Background of the Art
In most uses of silver halide in photographic materials, it is desirable to increase the speed or sensitivity of the emulsion. There are a number of different techniques for increasing the speed of an emulsion which are usually classified as chemical sensitization or spectral sensitization. Chemical sensitization usually involves modification of the silver halide grains to make the most efficient use of the radiation that they absorb. The three general types of chemical sensitization are sulfur sensitization, reduction sensitization, and precious (noble) metal sensitization. These methods of chemical sensitization are well known and firmly established in the art (e.g., James, T. H. and Vanselow, W. "Chemical Sensitization", J. Photo. Sci., 1, 133 (1953), Freiser, H. and Ranz, E., Ber der Bunsengesellschaft, 68, 389 (1964), and Pouradier, J. "Chemical Sensitization", Photographic Theory: Liege Summer School, A. Hautot, p. 111, Focal Press (London 1963).
Spectral sensitization enables grains to benefit from radiation in regions of the electromagnetic spectrum where the silver halide would ordinarily not absorb. Dyes which absorb radiation and can transfer energy to the grains to help in the photoreduction of silver ions to clusters of silver metal are conventionally used to effect spectral sensitization.
Another phenomenon associated with the use of spectral sensitizing dyes is known in the art as supersensitization. The addition of other substances, frequently in quantities ranging from an equivalent molar rate to a 100 fold molar excess of supersensitizer to dye, can increase the spectrally sensitized speed of the emulsion by more than an order of magnitude. Some supersensitizers are dyes themselves, but many others do not absorb radiation in significant amounts in the visible portion of the electromagnetic spectrum. Therefore, the effect of supersensitizers on spectral sensitization is not clearly dependent on the ability of compounds to absorb radiation in the visible portion of the spectrum. Certain cyanines, merocyanines compounds analogous to cyanines, certain acylmethylene derivatives of heterocyclic bases, and ketone derivatives such as p-dimethylaminobenzalacetone are known supersensitizers. An expanded selection of supersensitizers is therefore desired.
Mercaptotetrazoles are generally taught in U.S. Pat. Nos. 2,403,977; 3,266,897; and 3,397,987.
U.S. Pat. No. 3,457,078 describes the use of mercapto substituted oxazine, oxazole, thiazole, thiodiazole, imidazole or tetrazole as supersensitizers in combination with certain cyanine dyes.
U.S. Pat. No. 3,637,393 describes the use of mercaptotetrazoles in combination with certain hydroquinone compounds to reduce fog and increase speed in photographic emulsions.
The effects of supersensitizers are generally not additive as only so much speed increase can be obtained by the addition of speed enhancing compounds to silver halide emulsions. Each material needed usually is met by an effect more stringent than diminishing returns. In almost all cases multiple supersensitizers have been found to be limited to the maximum speed increase of a single supersensitizer. That is, the speed increase provided by combinations of supersensitizers is almost always no greater than that provided by the strongest supersensitizer.